Practice Problems II
(not in the lab manual; answers at the end of this page)

1. In turkeys, bronze body color (B) is dominant over red (b). Normal feathers (N) are dominant over hairy feathers (n). A bronze male turkey is mated to a bronze female, and some of the poults (baby turkeys) produced by this cross are red. What are the genotypes of the male and female parents and the red poults?

2. Some of the poults from the cross described in #1 above are bronze. What proportion of the poults would you expect to have the BB genotype? What proportion would have the Bb genotype? What proportion would have the bb genotype?

3. If you were to cross a BbNn turkey with a bbnn turkey, what proportion of the offspring will have red, normal feathers? What proportion will have bronze hairy feathers? red, hairy feathers? bronze, normal feathers?

4. Corn snakes (Elaphe guttata) are elegant creatures that are completely harmless to humans. Typical corn snakes have an attractive pattern of black spots on an orange background. However, some corn snakes have no black anywhere on their bodies, and look orange all over. Suppose you crossed an orange-and-black spotted snake with a solid orange snake and found that all the babies had orange and black spots. This would mean that the black-spotted allele was ___ to the non-black allele.

5. There are also corn snakes that never have any orange color on their bodies. These snakes may or may not have black spots, but the background color is nearly white, not orange. The orange allele is dominant to the white allele. If you found a corn snake that was white with black spots, what could its genotype be?

6. Suppose you crossbred two corn snakes that both had the genotypes OoSs. What proportion of the babies would be white all over (with no black spots)?

7. In parrots, gray color (G) is dominant over green (g). A second gene determines how dark the base color is (whatever the base color is: gray, green, or something else). The dark allele (D) is incompletely dominant over the light (d) allele, so that a bird with the Dd genotype has a medium-intensity color. What would a bird look like that had the genotype ggDD? What would a bird with the genotype GgDd look like? What genotype(s) would a light green bird have?

8. If you mated a light green parrot with a purebred dark gray parrot, what would the babies look like?

9. CHALLENGE PROBLEM. Suppose you took two of the baby parrots from the cross mentioned in question #8, and you crossbred them. How many kinds of offspring would you get, and in what proportions?


1. Since the problem doesn't say anything about what the feathers look like, you can leave out the N and n alleles. The red poults must have the bb genotype, because the bronze allele is dominant to the red allele. This means that the red poults must have received one b allele from the male parent, and one from the female. But since both parents are bronze in color, this means that both parents have to have the Bb genotype.

2. You'd expect to get 25% of your poults to have the BB genotype, 50% to have the Bb genotype, and 25% to have the bb genotype.

3. This one should be done with a Punnett square. The bbnn turkey will only contribute the allele combination bn to all her offspring. The BbNn turkey, however, can pass on four possible combinations of alleles: BN, Bn, bN, or bn. If done right, the Punnett square should show that all four phenotypes are equally probable. One quarter of all the offspring will be bronze normal; one quarter will be red normal; one quarter will be bronze hairy; one quarter will be red hairy.

4. Think here: either spotting is dominant to non-spotting, or it's recessive. If spotting is caused by a recessive allele -- let's call it s -- then the spotted snake has two such alleles: ss, and a non-spotted snake is either SS or Ss. In that case, if you crossed a spotted snake and a non-spotted snake, either half the babies would be spotted (if the cross is ss x Ss) or none of them would be solid (if the cross is ss x SS. But that's not what happens: the problem states that all the babies are spotted. This means that spotting can't be caused by a recessive allele. So it must be caused by a dominant allele.

5. If we let O be the dominant orange allele and o be the recessive white allele, then a white snake with black spots could be either ooSS or ooSs.

6. Straightforward Punnett square problem: you're crossing OoSs x OoSs, so set this up as a 4x4 Punnett square where each parent can pass on one of four allele combinations to its offspring: OS, Os, oS, or os. You should find that only 1/16 of the offspring would have the ooss genotype and thus be white without black spotting.

7. A ggDD bird would be dark green; a GgDd bird would be medium gray. For a bird to be light green, it would have to have two green alleles and two light alleles -- ggdd.

8. The light green parrot would have the ggdd genotype; the dark gray would have the GGDD genotype. The bouncing baby parrots would all have the GgDd genotype, and would all look medium gray.

9. Oooh, this one's eee-vil. . . . The cross is GgDd x GgDd, but what makes it tricky is the incomplete dominance of the darkness genes. You should do a 4x4 Punnett square, and if you've done it right, you'll get:

3/16 dark gray
6/16 (3/8) medium gray
3/16 light gray
1/16 dark green
2/16 (1/8) medium green
1/16 light green